A broadcast-type service includes both a broadcast and multicast services, and provides the same content for a plurality of user terminals existing within the cell of each base station at the same time. The user terminal receiving the broadcast-type service within the cell should be able to receive content with a certain quality of service (QoS) as far as it is located within the area. To that end, a sufficient receiving power needs to be provided for the respective user terminals.
The broadcast-type service is realized by a point to multi-point (PtM) system or a point to point (PtP) system.
In the PtM system, a base station uses a single point to multi-point channel (common channel) reaching the cell boundary to distribute content to all user terminals within the cell. The PtM system is also referred to as “common channel system”.
In the PtM system, a single downlink radio channel is used to distribute content to the user terminals, so that even in the case where a large number of the user terminals in the broadcast-type service area receive the content at the same time, the base station need not increase a transmission power of the radio channel irrespective of the number of the user terminals.
Therefore, with the PtM system, it is possible to increase the use effect of the radio resources in the case where a large number of the user terminals receive the broadcast-type service within the cell. When there is a sufficient downlink power capacity available in the operation of the broadcast-type service, it is possible for the base station to distribute content to all the user terminals within the cell by performing control such that the downlink transmission power of the PtM channel can reach the cell boundary. On the other hand, when there is not a sufficient downlink power capacity available, the base station performs control to set the downlink transmission power of the PtM channel to a value less than the downlink transmission power with which the PtM channel can reach the cell boundary. In this case, therefore, the base station can distribute content only to a part of the user terminals in the cell. That is, the user terminals near the cell boundary cannot receive the PtM channel and thereby cannot receive content.
In the PtP system, a base station uses point to point channels (individual channels) for respective user terminals to distribute the same content to the user terminals subscribing to the broadcast-type service within the cell. The PtP system is also referred to as “individual channel system”.
The PtP system is used when there are a comparatively small number of user terminals subscribing to the broadcast-type service within the cell. The PtP system uses the individual channel, so that the base station can set the downlink transmission power of the individual channel to an adequate value by performing transmission power control. Therefore, when the number of the user terminals is comparatively small, it is possible to reduce the downlink transmission power for distributing content to the user terminals.
As described above, the PtP and PtM systems are available for realizing the broadcast-type service. Here, in order to determine to use which of the PtP and PtM systems, it is necessary to make a determination based on the radio environmental capacity of the radio communication resources estimating the respective downlink transmission power.
Disclosed in a non-patent document (3GPP TR 23.846 1.2.0, September, 2002, P.17) is that the radio channel type (point to multi-point or point to point) is determined based on the number of the user terminals subscribing to the broadcast-type service within the cell.
In the case where the radio channel type is determined based on the number of the user terminals subscribing to the broadcast-type service within the cell as described above, it is considered that the PtP channel is used when the number of the user terminals is not more than a certain threshold value and the PtM channel is used when the number thereof exceeds the threshold value.
However, the above method of determining the radio channel type used for the content distribution based on the number of the user terminals subscribing to the broadcast-type service within the cell has the following problem.
For example, in the case where the single PtM channel is used to distribute content to the user terminals subscribing to the broadcast-type service within the cell, when the number of the user terminals changes to become not more than a certain threshold value, the radio channel used for the content distribution is switched from the PtM channel to PtP channel, and the PtP channel is used to distribute the content to the respective user terminals. However, if all the user terminals are located near the cell boundary, the total of the transmission power of the PtP channel for the user terminals becomes greater than the transmission power of the PtM channel.
That is, in some cases, the switching function of the radio channel used for the content distribution based on the number of the user terminals lowers, rather than increases, the use effect of the radio resources. Therefore, it is necessary to perform the switching based on the abovementioned radio environmental capacity of the radio communication resources.